This invention relates to an oil well completion fluid. More particularly, it relates to a fluid which comprises water, about 50 to about 65% by weight dipotassium phosphate, and at least about 10 ppm hexavalent chromium.
An oil well completion fluid is a fluid used in oil drilling to counterbalance the pressure of fluids in the earth, and prevent them from rising to the surface at the oil well except under the control of the oil well operator. Such fluids may be placed in between the casing and the piping or at the bottom of the piping. Because oil well completion fluids are in contact with the oil well piping, which is subject to corrosion, as well as in contact with the ground at the bottom of the well, a good oil well completion fluid must have a rather unusual set of properties that enable it to function well in this environment. These properties include a density of at least 12 lbs/gal., and preferably greater than 14 lbs/gal., so that the weight of the fluid keeps the oil and other fluids in the earth from entering the piping or otherwise coming to the surface except under the control of the operator. The completion fluid should be mechanically and chemically stable under the conditions encountered in the well; i.e., there must be no settling of suspended solids and no chemical precipitates if mixed with produced fluids or gases. Thus, it should have a crystal point temperature of less than 3.C as oil well drilling at sea and in some states is performed at low temperatures; a fluid that crystallized might also immobilize the piping and prevent it from being raised and lowered. It should also be stable at high (i.e., greater than 80.degree. C.) temperatures for long periods as some geological formations containing oil are in geothermal areas. Since the piping is generally made from carbon steel, the fluid must be relatively non-corrosive to carbon steel and should have a corrosion rate of less than 5 mils per year; the fluid should also not degrade to form compounds that are corrosive. The presence of organic material in the fluid is undesirable as organic matter can be easily thermally or bacteriologically degraded. The fluid should remain pumpable over the life of the formation and should not thicken or gel over time. An ideal fluid would form no significant solids as a result of exposure to up to 1800 ppm of calcium hardness, carbon dioxide, or hydrogen sulfide. The fluid should contain potassium cations rather than sodium cations because sodium cations tend to swell clay formations which plug the formations and impede the extraction of oil. A pH of less than 12 is desirable to prevent injury to the operators from handling caustic fluids.